How to thoroughly test a used Mac before buying

13 November 2015, 05:00

So, you’ve spotted a used Mac for sale. You want to make sure it works properly before handing over any cash, but you also don’t have hours to spend running through every possible test.

Here’s a few quick things to try.

Stress-test the CPU
To test to ensure the CPU works when it’s under full load, you can try the following.

  1. Open a Terminal window (you’ll find it in the Utilities folder of the Applications list in Finder), then type the following:
    sysctl -n hw.ncpu

    This tells you how many CPU cores the Mac has – both real and virtual, the former thanks to the fact Intel CPUs are dual-core and quad-core, and the latter thanks to hyper threading, which makes the Mac believe each core is actually two separate cores. The output of the command should be 2, 4, 6 or 8. On a Mac Pro you might even see 16. We need this info for Steps #2 and #3.

  2. In the Terminal window, type the following:
    yes > /dev/null

    This sends a stream of data to a dead end in your system, but does so as quickly as is possible. In fact, the command will entirely consume the CPU.

  3. The slight problem is that the command in Step #2 will only consume one of the Mac’s CPU cores, so you need to open a new tab in Terminal (Cmd+T) and then run the command again, in parallel with the first. In fact, you need to open as many tabs in Terminal as there are cores (as discovered in Step #1), each running the command. On my MacBook Pro I would need eight tabs, for example.

  4. Pretty soon the CPU fans will start to spin-up. Listen to them. They might whirr but they should sound smooth and not whiny. Let the commands run – and the fans spin – for five or ten minutes, to see how well the Mac copes with a difficult work load. When you decide the Mac’s had enough, just close the Terminal window.

If the Mac crashes during any of this, or freezes, or reboots unexpectedly, then there’s clearly a hardware issue. Walk away at this point because the issue will be difficult if not impossible to fix. However, be aware that the Mac might get a little sluggish while the tests are taking place, and hot too. This is perfectly normal. After all, the CPU is being used fully and there’s little computational power for other tasks.

Testing the RAM
Download Rember to the Mac. Once downloaded you’ll need to right-click it and select Open when you run it for the first time, because it’s a rather old app from 2010 that’s not been digitally signed in accordance with rules Apple’s introduced since.

However, despite its age Rember runs well enough for our purposes. It’s a graphical version of the classic memtest86 memory testing app. It writes data patterns into every location of memory. Just click the Start button on the app interface and watch the progress bar. Errors will be reported if/when they occur, or you can click the Log tab to watch output live as it happens. Click Verbose Logging for even more info.

There’s some question about whether, as a GUI app running on top of Mac OS X, Rember can actually access every memory location of your RAM. But it’s the best choice for a quick test in this situation, and will certainly give the system RAM a good workout.

If you experience crashes or freezes then, again, walk away. Even though this might indicate “just” a RAM issue, and RAM can be swapped in older Macs (although NOT most newer Macs), it could also indicate a problem with the CPU or motherboard. It’s just too risky to take the chance.

Checking the hard disk/SSD
Unlike Windows a Mac doesn’t have built-in surface sector (block) checking. You can buy this in the form of Drive Genius or Disk Warrior but we’re keeping things quick and simple here, so need another method of testing the disk.

Start with a permissions scan. This checks system files for various things unrelated to whether the disk’s sectors are all in-tact, but doing so can sometimes unearth read errors.

On versions of OS X prior to El Capitan, you can simply open Disk Utility (it’s in the Utilities folder of the Applications list) and click the Repair Permissions button. On El Capitan and later you’ll need to use a trick detailed in an earlier Mac Kung Fu posting.

Don’t worry if the scan comes back saying some files have incorrect permissions (typically “user differs…” or “group differs…” messages in the output). This is entirely normal. What you’re looking for are read errors, or anything indicating difficulty in actually accessing a file, or system freezes and crashes.

Shut down the Mac once the test has finished, then boot into Recovery Console by tapping Cmd+R before the Apple logo appears. Start Disk Utility by selecting its entry on the menu, then select to Repair Disk. You might see error messages but hopefully they should be repaired. If you see an error message saying the issue can’t be repaired then something might be seriously wrong – although it’s equally possible the disk file system could be corrupted and a reformatting of the disk and subsequent reinstallation of OS X will fix things.

Again, you’re looking for suspicious activity – read errors reported in the output, or freezes that clearly indicate difficulties. However, be aware that during scanning the machine may become unresponsive. This again is not unusual.

Some people advocate checking a disk’s SMART status, wherein the disk reports if it’s close to failure. In my experience this is mostly useless – dodgy disks report they’re absolutely fine – but instructions are available online and I mention it here for the sake of completeness.

Testing the sound and screen
You can look for dead pixels on the screen by visiting this YouTube video, turning the screen brightness up full, and then switching the playback window to full-screen mode.

Be aware that some Macs may have had a few dead pixels since they were manufactured, so spotting some isn’t necessarily a sign of anything wrong – or that anything is going to go wrong. However, Apple’s threshold was two or three dead pixels at most during manufacture, so if you’re seeing more then there could be an issue with the display.

As for testing the speakers, well, just play some music! Turn it up! Some Macs might distort at full volume but this shouldn’t be too severe and should be on the pleasant side of distortion, rather than the unlistenable side of things.

Apple Hardware Test/Apple Diagnostics
You can hold down the D key while booting the Mac to use either Apple Diagnostics, if the Mac was created after 2013, or Apple Hardware Test, for older Macs. This will run through a series of hardware tests. It’s worth undertaking, although the tests aren’t considered particularly thorough.

The drop test
An old test when buying any used computer is to TURN IT OFF and then lift it from the surface by a centimetre or so, before letting it drop back down. This will test for lose connections inside. You probably want to ask first before doing this, though…



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